This invention relates to thin film transistors, and more particularly to semiconductor thin film transistors having reduced photo-induced current, improved electrical contact between a transparent electrode and the source or drain region of the transistor, and reduced shorting between a source line and a gate line in a matrix array, resulting in improved liquid crystal display devices including the transistors.
The formation of a thin film transistor (hereinafter referred to as TFT) on an insulating layer has applications to many fields. TFTs can be used in thin active matrix display panels formed on inexpensive insulating substrates, three-dimensional integrated circuits having an active element such as a transistor formed on a semiconductor integrated circuit, inexpensive, high-performance image sensors, high-density memory devices, and the like.
Thin active matrix displays utilizing TFTs include an upper transparent substrate, a lower transparent substrate on which the TFTs are formed and a liquid crystal material sealed between the substrates. Liquid crystal driving electrodes are arranged in the matrix of TFTs. An external selecting circuit selects some of the liquid crystal driving elements and each associated liquid crystal driving electrode is excited to display the desired letters, figures or pictures. When the TFTs are formed on an inexpensive insulating substrate and a transparent electrode of a material, such as indium oxide, tin oxide, indium-tin oxide, or the like is used as the display driving electrode, a transparent-type display panel is obtained.
Large-scale liquid crystal display devices utilizing picture elements arranged in an active matrix are currently attracting much attention. They are now being used in a variety of applications, such as small-size personal computers, hand-held televisions and the like. In such display devices using picture elements arranged in the matrix array, the use of switching elements for, in particular, planar, large scale display devices is currently being developed. When a TFT is used in an active matrix panel, each TFT controls application of the voltage of the data signals to the liquid crystal material. In order to obtain superior display performance, the TFT must have the following characteristics.
(1) When the TFT is in the ON condition it must supply sufficient current to charge the capacitor. Improved display performance depends on the potential of the capacitor, which must be stored in a short period of time.
(2) When the TFT is in the OFF condition, it exhibits low current leakage. This is necessary as the charge in the capacitor must be held for a longer period of time than the writing period.
When a TFT is irradiated with light, carrier density in the accumulation region is increased, due to the light. The depletion layer at the PN junction is narrowed by the increased carrier density. As a result, both ON current and OFF current are increased. The increase in OFF current is significant. In fact, the increment of leakage current caused by irradiating light is proportional to the illuminance of light. Thus, the brighter the environment is made, the more OFF current increases. While the contrast and display characteristics of a liquid crystal display device generally improves in bright light, the same light simultaneously deteriorates the TFT display performance, due to the increase in OFF current caused by light. Therefore, the TFT have this disadvantage when used in a liquid crystal display device as a switching element.
A TFT in a matrix display device has a three layer electrode construction for connecting a drain electrode and a driving electrode. This conventional construction is not fully satisfactory as the metallic layer connecting the drain and electrode can form an insulating oxide such as, for example, aluminum oxide when an aluminum layer is used to couple the drain and the driving electrode. As a result, unwanted variations in display contrast are caused by the insulating effect of the oxide on the contact portions, thereby adversely affecting the display characteristics.
After the matrix array is completed, a breakdown of insulation between lines tends to occur due to static electricity or the like. When static electricity is applied to a source line or gate line outside of the display area, poor insulation results at the intersection of orthogonally crossing lines. As a result of this poor insulation, a data signal leaks into a gate line or a timing signal leaks into a source line. This leak occurs at the intersection of the gate line and source line, which is the so-called line fault. As a result, the display by every picture element connected to the line fault is affected adversely and the display characteristics of the matrix array deteriorate substantially.
The most general method for compensating for deteriorated insulation is cutting off the source line or the gate line around the poor insulating portion. In this method, however, a line fault inevitably occurs as the elements connected to the broken source line or the broken gate line are in a non-lighting condition. In the case where a matrix array is formed on a single crystalline silicon substrate, static electricity can be avoided by providing a diode or other resistance in the silicon substrate. On the other hand, in the case where a matrix array including thin film transistors is formed on a glass substrate, poor insulation is likely to occur, because it is very difficult to provide a circuit for warding off the invasion of static electricity. As a result, the production yield of the matrix array is remarkably unsatisfactory due to poor insulation. Accordingly, there exists a need for improved thin matrix arrays and display devices, including the active matrix arrays which have reduced photo-induced current, improved electrode contact particularly in the case of transparent driving electrodes coupled to the drain by a metal which forms an insulating oxide film, and have improved insulation between the source line and gate line or the driving electrode, particularly in the case of TFT arrays formed on an insulating substrate.